Course Content
Crop Production (Unit 6)
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ASRB NET / SRF / Ph.D. Agronomy
Characterization of the Agro-Climatic Environments of Drylands
  1. Defining the Dryland Agro-Climatic Environment

Drylands are not defined merely by low rainfall, but by:

  • High variability and uncertainty in precipitation.
  • High atmospheric demand for water (evapotranspiration).
  • Permanent moisture deficit, which dictates specific farming systems.

Core Defining Parameter: Moisture Availability

  • Aridity Index (AI): Ratio of Annual Precipitation (P) to Potential Evapotranspiration (PET).
    • Dryland Agriculture occurs in regions with AI = 0.20 – 0.50.
    • This implies that only 20–50% of the water lost to the atmosphere is replenished by rainfall.

 

  1. Key Characteristics of Dryland Agro-Climates

(i) Low and Erratic Rainfall

  • Fact: Drylands receive <750 mm annual rainfall (arid zones <500 mm).
  • Implication: Insufficient for most crops without soil moisture conservation.
  • Critical Point: Rainfall reliability is a greater challenge than total rainfall. Example: A region may average 600 mm, but receive 300 mm one year and 900 mm the next.

 

(ii) High Temporal and Spatial Variability

  • Temporal (Within a Season): 70–80% of rainfall may occur in just 20–30 rainy days during monsoon. Implication: Narrow sowing window; delays of even one week can cause crop failure.
  • Spatial (Across a Region): Rainfall is highly localized (e.g., scattered thunderstorms). Implication: Makes large-scale drought management difficult.

 

(iii) High Temperature and Evapotranspiration (ET)

  • Fact: Drylands face high temperature, intense solar radiation, and low humidity.
  • Implication: PET often >1500–2000 mm/year, much higher than rainfall.
  • Example: Rainfall = 600 mm; PET = 1800 mm → severe moisture deficit.

 

(iv) Frequent Drought Occurrences

  • Fact: Meteorological droughts are common. In India, drought occurs every 2–5 years; in Rajasthan, almost every alternate year.
  • Implication: Drought is a recurring feature, not an exception → risk mitigation is essential.

 

(v) Soil Characteristics

  • Low Organic Matter: <0.5% due to sparse vegetation and high decomposition.
  • Erosion-Prone: Intense rains + poor ground cover → wind & water erosion.
  • Shallow Depth: Many regions (e.g., Deccan Plateau) have shallow soils with hardpan/rock → limits root growth & water storage.

 

  1. Agro-Climatic Zoning of Drylands in India

Agro-Climatic Zone

Key Features

States

Farming Systems & Crops

Western Dry Region

Arid (<300 mm), High ET

W. Rajasthan, N. Gujarat

Dry farming, fallowing, water harvesting. Crops: Pearl millet (Bajra), Cluster bean (Guar), Moth bean.

Deccan Plateau

Semi-Arid (500–750 mm), Shallow soils

Maharashtra, Telangana, N. Karnataka

Contour bunding, intercropping. Crops: Sorghum (Jowar), Sunflower, Chickpea (Chana).

Central Plateau

Semi-Arid (700–1000 mm), Problematic soils

MP, E. Rajasthan, UP

Watershed management. Crops: Soybean, Pigeon pea (Arhar), Wheat (residual moisture).

Rain Shadow Region of Eastern Ghats

Semi-Arid (600–850 mm)

Tamil Nadu, Andhra Pradesh

Dryland farming. Crops: Groundnut, Pulses, Millets.

 

  1. Summary: Core Features of Dryland Agro-Climate

Feature

Description

Impact on Agriculture

Precipitation

Low (<750 mm), Erratic, Unpredictable

High risk of crop failure; dictates sowing time

Aridity Index

0.2 – 0.5

Permanent soil moisture deficit

Evapotranspiration

Very High (>1500 mm/year)

Rapid loss of stored soil moisture

Drought Probability

High (every 2–5 years)

Farming must include risk mitigation

Soil Quality

Low organic matter, shallow, erosion-prone

Requires soil conservation & fertility management

 

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